This invention broadly relates to ion-selective electrodes, and more particularly to reference probes for potentiometric measurement of ion-concentrations in a test solution.
Metals have long been known to contaminate aquatic environments. Copper, for example, often leeches from marine paints into harbors and causes aquatic contamination. Copper ion concentration profiles are useful to precisely determine the extent of copper contamination. With contamination levels known, effective remedial measures can be chosen. Accordingly, deep water devices have been developed which profile contaminate concentrations in sea water. Such devices may rely on potentiometric measurement techniques.
Simple potentiometric measurement systems typically employ an electrode pair immersed in a test solution. The pair includes a reference electrode and a working electrode. In operation, a minute current (e.g. 10.sup.-9 amps) is drawn between the electrode pair for an instant (e.g. 1 second) to make a voltage reading. The amount of charge drawn is so small that ion concentrations in the test solution are not significantly affected. The measured voltage potential between the electrode pair indicates ion concentrations. Importantly, the reference electrode maintains a stable potential to enable meaningful measurements by the working electrode.
A typical reference electrode includes a housing for holding a reference element in an electrolyte. This arrangement forms a half-cell which provides the reference potential needed for meaningful potentiometric measurements. Ionic communication is permitted between the electrolyte and the test solution through a junction in the housing.
The most common types of ion-selective reference electrodes utilize a reference element made from silver-silver chloride (Ag/AgCl) or mercury-mercurous chloride and a chloride-based electrolyte such as a saturated aqueous solution of potassium chloride. One such reference electrode is disclosed in U.S. Pat. No. 3,705,089 by Grubb. Such electrodes form measurable amounts of solids (e.g. silver chloride) during operation from ions dissolved in the aqueous solution. Reactions involving solid formations, however, are notoriously slow and chemically irreversible. Additionally, electron drift and hysteresis resulting from pressure and temperature changes reduces measurement accuracy. A significant junction potential between the electrolyte and test solution further limits measurement accuracy.
Noteworthy attempts to increase measurement accuracy by reducing hysteresis due to temperature and pressure changes manifest themselves in U.S. Pat. No. 4,495,050 by Ross and U.S. Pat. No. 4,959,138 by Brinkmann et al., respectively. Both inventions, however, rely on a chloride based electrolyte. Ionic measurements taken with these devices rely on solid formation. As a consequence, ionic reactions are irreversible and ionic measurements are not optimally accurate.
The present inventive concept overcomes the above limitations by providing a reference electrode probe which employs a specially adapted electrolyte. The electrolyte does not depend on solid formation. Instead, ionic activity in the electrolyte is freely reversible. The electrolyte also includes a solvent formed from a portion of the test solution to minimize associated junction potential and increase measurement accuracy. With these modifications, the present invention is particularly suitable for use with accurate potentiometric profiling of ion concentrations in the ocean.